A question on flash exposure

Hi all, hope someone can help. I have a d7000 and a SB800 speedlite. When I attempt to take a picture where the ambient light is low using, say F4 and a shutter speed of 125 the light meter, as viewed through the view finder, tells me that the image is going to be under-exposed. However the resultant image is exposed perfectly. I believe the SB800 puts out a pre-flash in order to evaluate the scene. Does the SB800 then, for want of a better word, override the camera's shutter speed setting or in some other way bring the exposure into balance?

Re: A question on flash exposure

Without seeing a image that you took there are many variables when using flash. If you want the ambient exposure to be correct you should set the exposure in the camera for the ambient light whether the ambient light is dark or daylight so the image if you took it without the flash would come out ok. Then you turn on your flash and your right the flash sends a pre flash a nano second before the main flash goes off, it looks like it's just one flash but there are two. I hope this answers your Q, there is a lot to learn when using flash.
Dave.

Re: A question on flash exposure

Your meter is reading the scene without the benefit of the flash light hence the under-exposure warning. When you fire the flash , and assuming you have the SB800 set to TTL metering then the camera and flash work in tandem to produce the correct exposure. The camera meter reads the light value and automatically adjusts the light from the flash. This will only work if you are working within the capabilities of the flash. Try taking a photograph with your flash in a very large dark room at low ISO and the back of the room will be dark due to its lack of power.
I have an SB800 and have found it a very reliable performer.

Re: A question on flash exposure

If you're shooting with iTTL, what's happening is that your camera is telling your flash to put out a small "preflash" burst of light of a known brightness. The camera meters the "preflash" through the lens (TTL), and then adjusts the power on the flash according to the reading for the main flash burst during the exposure. If the flash burst metered below the known brightness level, the power setting is increased accordingly, and if it metered higher than the known brightness level, the power setting is decreased.

So, whatever gets lit by your flash will probably have good exposure, or at least be in the ballpark of good exposure.*

The background that isn't getting hit by the light from your flash may be a different story.

* with TTL, because it's a metering-based way of changing an exposure setting, it won't be 100% foolproof in all situations. Metering can be fooled by the contents of a scene. So, you do want to know how to use FEC (flash exposure compensation) to adjust when the metering's thrown off. Just as you use A mode on the camera for speed & convenience and M mode for control & consistency, you use iTTL on a flash for speed & convenience and M for control and consistency.

Re: A question on flash exposure

Thanks to all of you for taking the time to help me understand a bit more about what is happening with the flash. The explanations you have given make perfect sense but, until now, I could not find a clear explanation written down anywhere. Many thanks again, James.

Re: A question on flash exposure

James while the other three posters are right in what they are saying they still haven't answered your Q. If you are taking a image of somebody at night in a city street and they are in front of a partly lit building if you use TTL it will send out a pre flash as I said and it will note that there is not much light. It will send out what it thinks is the right amount of flash. The subject will be brightly lit and everything behind the subject will be in darkness Meter for the background with your flash off and then take the image with the flash on and you will be able to see the building behind and your subject won't be overblown. The best way to learn how to use your flash is to get to know by using it on manual and do that in a darkened room and set your flash to manual and use it in different strengths 1/4 and so on.

Re: A question on flash exposure

Hi again Dave. All noted regarding your comments. What you are saying, if I understand you correctly, is that by first metering the ambient background light the flash, using the information supplied by the camera, will then adjust its output to expose the foreground correctly, without it being overblown. I have experimented a little bit, in the past, along the lines you have suggested. The only thing that bothers me is that with low ambient light the f-stop is usually wide open (very shallow depth of field) and the shutter speed is at a point where camera shake is a serious possibility! I know you will tell me, if I am really serious, to use a tripod. However that solution is not always possible. I have been experimenting hand-held with a shutter speed of 1/125s and an aperture of f5.6 and letting the flash do what it does best. The results have been acceptable. However the mystery of what part the flash played in the whole equation led me to post my initial question. I am learning, but slowly! Thanks again Dave.

Re: A question on flash exposure

James, one thing to keep in mind when using flash is that the flash via aperture controls the exposure on the subject, while the shutter controls the exposure on the ambient light. In essence you are dealing with two (not one) exposure variables.

Re: A question on flash exposure

Joe McNally did a lecture in Melbourne on this topic. You can find it all on YouTube if you search for "Flash photography with Joe McNally close up at the digital show" parts 1-4. For your question, the most relevant portion (part 2) is below. Pay special attention to what he does with his flash settings when shooting the bearded gentleman.

The critical factor is that Nikon's (but not Canon's) flash system meters flash and ambient lights independently. IE, if you have a flash installed and you set the camera exposure to -1EV (or whatever), the flash power will increase to compensate. So you can darken the background with the camera's exposure compensation, while keeping the foreground/subject lit properly. To brighten or darken the foreground/subject, you change the flash's exposure compensation. Again, this applies only to late-model Nikons, but the principle of balancing foreground and background lights applies universally.

Re: A question on flash exposure

Originally Posted by Mingmoos

I have been experimenting hand-held with a shutter speed of 1/125s and an aperture of f5.6 and letting the flash do what it does best. The results have been acceptable. However the mystery of what part the flash played in the whole equation led me to post my initial question. I am learning, but slowly! Thanks again Dave.

James, now assume that you don't have a subject to use the flash, instead you are shooting only what would be the background. If there is a chance that 1/125s, f/5.6 at whatever ISO you have selected, will result in an underexposure, the flash will not address it. For this situation, manual exposure may be a good idea.

Re: A question on flash exposure

I think I am beginning to get my head around the issue. As the flash only lasts about 1/1000s and the maximum synced shutter speed I can use with my D7000 is 1/250s then the flash is long gone (so to speak) before the shutter finally closes. Therefore the shutter speed has no affect on the flash exposure. With the flash gone the only thing that the shutter speed can affect (in the remaining time) is the ambient light which is a constant. A faster shutter speed (up to the sync maximum) will allow less of the ambient light to appear in the background and a slower speed more of the ambient. As you point out Bruce there are actually two exposures, the flash plus the aperture being one and the shutter plus the ambient light being the other! In all of the above I am assuming a sensitivity of ISO100. Thanks again to everyone for their input, it has been most helpful. Any further thoughts on the subject would be appreciated.

Originally Posted by Digital

James, one thing to keep in mind when using flash is that the flash via aperture controls the exposure on the subject, while the shutter controls the exposure on the ambient light. In essence you are dealing with two (not one) exposure variables.

Re: A question on flash exposure

You might also want to have a look at this contemporaneous thread (it's an old one but was resurrected again from post #34 onwards). It sort-of adds to the picture in terms of the issue being addressed in this thread

Re: A question on flash exposure

Thanks for pointing me in the direction of the old posting Donald. I can see that the discussion on high speed sync adds a completely new dimention to what I have already learned. Looks like I have a long way to go on the learning curve!

Originally Posted by Donald

You might also want to have a look at this contemporaneous thread (it's an old one but was resurrected again from post #34 onwards). It sort-of adds to the picture in terms of the issue being addressed in this thread